The present invention relates to multilayer reflective films, particularly to reducing stress in substrates containing multilayer films, and more particularly to depositing a stress free multilayer system which contains two different material combinations of opposite stress, and depositing the material combination with higher reflectivity last, with a smaller total film thickness than in prior buffer layer arrangements.
Multilayer structures composed of alternating thin layers of materials with different optical properties, such as molybdenum (Mo) and silicon (Si), and molybdenum (Mo) and beryllium (Be), have proven effective as high-reflectance, near-normal incidence coatings for various applications. The Mo/Si and Mo/Be systems, for example, which have a high reflectance (about 60 percent) for certain wavelengths (10-25 nm) is of particular technological importance in high-resolution, multiple-reflection imaging systems.
Extreme-ultraviolet (EUV) lithography systems, for example, require several precisely figured, low roughness optics coated with highly reflective multilayers. To obtain sufficient throughput and image quality, these multilayer coatings should simultaneously have high reflectance (at least 65%) and low residual stress (less than 100 MPa).
An optic (substrate) will deform when a stress multilayer film is deposited upon it. A viable EUV lithography process will rely on Mo/Si or Mo/Be multilayer films to effectively reflect light in the 11-14 nm wavelength region. Mo/Si and Mo/Be films with high reflectances (greater than 60%) have large film stress (at least -400 and +330 MPa, respectively), which will deform the optic and potentially degrade the performance of an EUV lithography tool. Thus, there is a need for reducing the stress in the multilayer films without adversely effecting the reflectance of these films. Reflectance is importance since the throughput of an EUV lithography system is a function of the reflectances of each optic.
Recently a non-thermal approach to producing multilayer reflective films or coatings with high reflectance (greater than 60%) and low stress (less than 100 MPa) was developed, which utilizes a buffer layer between the multilayer film and the substrates, the buffer layer film being of opposite stress to the stress of the multilayer film. This non-thermal approach is described and claimed in copending U.S. application Ser. No. 09/027,309 filed Feb. 20, 1998 entitled "Method To Adjust Multilayer Film Stress Induced Deformation Of Optics", and assigned to the same assignee.
The present invention extends the non-thermal approach of the above-referenced application by using a stress reducing approach that simultaneously reduces/compensates stress without a significant degradation in reflectivity, and at a smaller total film thickness than for the earlier buffer-layer approach. This approach makes it possible to achieve near-zero stress, or adjust the stress, with high reflectivity and smaller total film thickness than was required in the approach of the above-referenced application. A key feature of this invention is the smaller total film thickness. The present invention provides stress free multilayer systems which contain multilayer systems with two different material combinations (e.g., Mo/Be and Mo/Si) of opposite stress, where both systems give good reflectivity at the design wavelengths. If the optical performance of the two systems (Mo/Be or Mo/Si, for example) at the design wavelength differ, the system with the poorer performance is deposited first, and the system with the better performance is deposited last, thus forming the top of the multilayer reflective film or coating. The switch-over point from one system to the other system is determined by the relative stress in each system and is at about half of the total thickness of the multilayer stack for Mo/Be--Mo/Si.